2013 Ag Retail Outlook: Rethinking Soil Sampling

Anyone not directly involved in U.S. production agriculture could easily believe annual crop yield increases could go on year after year. The introduction of hybrid seed corn and commercial fertilizer were certainly the early factors in yield increases. But now, genetic improvements and the use of precision farming technologies continue to improve our ability to increase yields while at the same time our soil test nutrient values continue to decline.  


We are mining our soils, taking off more nutrients each year than we are replacing. With the increase in crop residue removal, the trend in lower soil test levels could accelerate even more if we don’t rethink our nutrient application rates.

During the winter of 2012, I reviewed the results of more than 100,000 grid soil sample results taken by Midwest Independent Soil Sampling (MISS) in Iowa in the fall of 2011. I found 41% tested at 20 ppm or less in phosphorous. Potassium levels were even worse, with more than 50% testing at 170 ppm or less in plant-available potassium.  

The results of the fall 2011 Iowa samples parallels the findings in other Midwestern states by the International Plant Nutrition Institute (IPNI) in a study of more than 4.4 million soil sample analyses across North America in 2010. This extensive study is available from the IPNI, Publication No. 30-3110, and compares soil survey results from 2010, 2005 and 2001.  Using the Iowa results from the IPNI the study found 46% of the Iowa soil test phosphorous values were at the critical level of 20 ppm or less, and 55% of the potassium soil analysis results were at the critical level of 170 ppm or less. The results are very similar to the MISS findings.

The IPNI study reveals that it is the better, more aggressive agronomy managers and farmers who regularly sample their fields and the results may not be representative of those who do not sample. Further, both data sets include soil test results taken from fields under livestock nutrient management plans, and in all probability resulted in higher soil test values.

I believe those who do not regularly sample are much more likely to have lower fertility levels overall, as they are not involved in site-specific application based on soil test values and actual site-specific crop removal levels and aren’t applying livestock nutrients. Nutrient applications based on average field yields leads to less than average yields over the long run, and an even worse decline in soil test values.

Another indication we are mining many of our soils in the Midwest are the early stages of potassium deficiencies showing up in specific areas of fields.  The blame last year was placed on the early-season dry weather of 2012 which certainly didn’t help, especially with nutrients such as potassium. But when we see four- to six-inch corn in Iowa showing signs of K deficiency, we need to examine our sampling intensity, soil test values and fertility recommendations. Our critical levels of K in this case are simply too low.

Today’s producer is adopting precision tools at an unbelievable rate, but many are centered on equipment that is offering valuable operational improvements. Seed placement, seed spacing, population, variable rate planting and genetics improvement are all factors leading to an increase in yield. When these advancements level off (and they will), yields will level off and will actually decline if we don’t make better nutrient application decisions.   

At the same time we are seeing and will continue to see extreme pressure by regulatory agencies to improve water quality. Agronomy service providers will be called upon to become a more important part of this process while at the same time helping producers to maintain and increase crop production. Applying the 4Rs of fertility management is an extremely useful tool but it comes with the responsibility to build and maintain our soil fertility at levels that maximize and protect our most valuable resource, our soils. We have the science and the tools to accomplish all of the challenges facing agriculture, all of which start with the soil.